This invention relates to weatherable structures and a process for preparing said structures. More particularly, this invention relates to laminated structures useful as materials for the building and construction industry. The weatherable structures of the present invention have reduced stress induced curl or surface distortion ("oil canning").
Currently, residential home vinyl siding having weatherable cap coatings are produced by coextrusion of a relatively thin weatherable cap layer over a thicker non-weatherable (and lower cost) substrate. For example, for light colors such as white or beiges weatherable poly(vinyl chloride) (PVC) is typically extruded over a non-weatherable PVC substrate. For darker colors, such as brown, red, blue, gray or green, PVC generally does not exhibit adequate weather stability, so alternate weatherable polymers such as AES (acrylonitrile-ethylene/propylene rubber-styrene) graft copolymers or ASA (acrylonitrile-styrene-acrylate rubber) graft copolymers are coextruded over the PVC substrate. U.S. Pat. No. 4,438,171, for example, discloses preparing a thick skeet laminate by coextruding a layer of an AES based on dicyclopentadiene (DCPD)-type EPDM onto a thermoplastic resin such as ABS (acrylonitrile-butadiene-styrene) or PVC.
In the preparation of weatherable structures by coextrusion, for example vinyl siding for the building and construction industry, there is a continual problem with the presence of "oil canning". "Oil canning" is warping, rippling or other surface distortion of a structure caused by unequal stress within the substrate imparted during extrusion.
Oil canning is prevalent in the coextrusion of AES and ASA-type polymers over PVC because AES-type polymers (such as ROVEL.RTM. brand weatherable polymers manufactured by The Dow Chemical Company) and ASA polymers (such as Geloy.RTM. and Centrex.RTM. weatherable polymers manufactured by GE and Monsanto, respectively) generally exhibit higher melt viscosities than the PVC and are typically processed at higher melt temperatures than PVC (temperatures which are high enough to thermally degrade PVC). As a result, coextrusion of a uniform AES or ASA cap coat is difficult to achieve.
A normal gauge thickness for a weatherable cap coat, such as AES, is typically desired to be at 5 mils. However, coextrusion of AES over PVC is difficult due to viscosity mismatch, and thus in preparing vinyl siding the siding product may be coextruded with a cap coat gauge ranging from 4 to 12 mils across the siding surface. Even though a 5 mils coating is typically the target, a nominal 8 mil (.+-.4 mil) coating must be extruded in order to insure that at least 4 mils of cap covers the entire product. Not only does this significantly increase raw material costs, but the non-uniform cap coating imparts undesirable stresses upon the PVC substrate. These stresses are believed to cause the very undesirable "oil canning" which is curling or warping of the siding.
A study of the effects of polymer properties such as the coefficient of thermal expansion, heat distortion temperature, and flexural modulus (stiffness) upon the dimensional stability of exterior building products such as residential siding and window and door profiles is discussed by Elvira B. Rabinovitch in Journal of Vinyl Technology, Volume 10, No. 3, Mar. 1988, p. 14 et seq. The study showed that the coefficient of thermal expansion of a material (as well as the heat build-up due to solar radiation absorption of darker colors) was a primary factor affecting vinyl siding dimensional stability (oil-canning). Additionally, residual mechanical stresses within a siding structure due to material processing inconsistencies or non-uniformities are also a major factor dimensional instability.
Rubber modified styrenic polymers, such as AES and ASA, exhibit substantially different polymeric properties from those of exterior rigid PVC compounds. Typical key physical properties are shown in the following Table I:
TABLE I __________________________________________________________________________ COMPARISON OF KEY PHYSICAL PROPERTIES OF SIDING RESINS ASTM Exterior Exterior Test PVC Normal PVC High Property Units Method AES.sup.(1) ASA.sup.(2) Impact.sup.(3) Impact.sup.(3) __________________________________________________________________________ Specific Gravity -- D792 1.02 1.06 1.45 1.42 Flexural Modulus psi (105) D790 3.0 2.4 4.3 4.1 (MPa) (103) -- (2.07) (1.69) (3.0) (2.83) Heat Deflection .degree.F. D648 210 190 160 162 Temp. @ 264 psi (.degree.C.) -- (99) (88) (71) (72) Coef. of Linear in/in/.degree.F.(10.sup.-5) D696 4.4 5.9 3.6 3.5 Thermal Expansion (cm/cm/.degree.C.) (10.sup.-5) -- (7.9) (10.6) (6.5) (6.3) __________________________________________________________________________ .sup.(1) AES = ROVEL.TM. 501, trademark of The Dow Chemical Company .sup.(2) ASA = GELOY.TM. 1120, trademark of The General Electric Company .sup.(3) PVC = GEON.TM. PVC's, trademark of B. F. Goodrich Company
The AES and ASA weatherable polymers exhibit substantially lower specific gravity and modulus (stiffness) and higher heat deflection temperature and coefficient of linear thermal expansion than rigid PVC.
As a result of these inherent property differences, multilayer structures produced by non-uniform coextrusion of dissimilar polymers, such as is frequently experienced with AES or ASA coextruded over rigid PVC, can result in significant incorporation of stress within the extruded structure. These gauge related internal stresses coupled with temperature induced differential thermal stresses (particularly within a complex profile shaped structure) can result in siding surface distortion or oil-canning.
Oil canning can be seen during production and most notably in the field during siding installation or on the house once installed. It is estimated that as much as 20% of siding produced by coextrusion of dissimilar resins exhibits oil canning in a typical plant and must be scrapped (or recycled). Additionally, oil canning of siding product produced from coextrusion of dissimilar polymers is the predominant reason for customer complaint and product return.
Another disadvantage of a coextrusion process is that such a process requires expensive capital investment (second extruder, second resin feed system, coextrusion die), and as with any extrusion, color changes requires significant polymer purging, which increases overall production costs.
Another method of preparing multilayered structures is to laminate a cap coating or layer in the form of a previously prepared film onto a separately formed substrate layer. Some lamination processes have the disadvantage of requiring the use of an adhesive layer between the weatherable film and substrate for adhering the film layer onto the substrate because the film layer may be difficult to adhere to a substrate without an adhesive. For example, U.S. Pat. Nos. 4,680,234 and 4,724,186 disclose a multilayered laminated structure employing an adhesive layer to bond a film to a substrate.
Difficulty occurs when laminating weatherable polymers such as films produced from acrylics (Korad.TM. polymers manufactured by Polymer Extruded Products), or fluoropolymers (Kynar.TM. and Tedlar.TM. polymers manufactured by Atochem and E. I. du Pont de Neumours, respectively) which are incompatible or immiscible with a PVC substrate. Thus, an adhesive must be used to bond these types of polymer layers to PVC. Additionally. these incompatible polymers also present difficulties with in-plant recycle and re-extrusion of recycle-containing substrate layers and can result in structures with inferior physical properties.
Various patents disclose laminating an AES-type film onto various substrates other than PVC siding. For example, U.S. Pat. No. 4,444,841 describes laminating an extruded AES film onto a metallic or cellulosic (i.e. wood) base material; U.S. Pat. No. 4,444,840 describes laminating a calendered AES film onto an ABS substrate and U.S. Pat. No. 4,440,825 describes laminating a sheet of an AES graft copolymer composition layer onto a sheet layer of foamed ABS or foamed PVC to form a sheet product.
Still there are other patents, for example, U.S. Pat. Nos. 4,169,180; 4,183,777 and 4,100,325, which disclose preparing a laminate useful as a construction material such as residential siding comprising a cross linked acrylic or plasticized PVC protective layer on a PVC or other rigid (such as ABS) substrate. However, these patents do not address the problem of oil canning.
It is therefore desired to provide a process for preparing a laminate structure by laminating a cap coating or outer layer onto a vinyl substrate without the need for adhesive layers and wherein said laminates exhibit reduced oil canning as compared to corresponding structures prepared by coextrusion processes.